Crystal structures of the nitrite and nitric oxide complexes of horse heart myoglobin

Nitrite is an important species in the global nitrogen cycle, and the nitrite reductase enzymes convert nitrite to nitric oxide (NO). Recently, it has been shown that hemoglobin and myoglobin catalyze the reduction of nitrite to NO under hypoxic conditions. We have determined the 1.20 angstrom resolution crystal structure of the nitrite adduct of ferric horse heart myoglobin (hh Mb). The ligand is bound to iron in the nitrito form, and the complex is formulated as Mb(III)(ONO-). The Fe-ONO bond length is 1.94 angstrom, and the O-N-O angle is 113 degrees. In addition, the nitrite ligand is stabilized by hydrogen bonding with the distal His64 residue. We have also determined the 1.30 angstrom resolution crystal structures of hh Mb(II)NO. When hh (MNO)-N-II is prepared from the reaction of metMb(III) with nitrite/dithionite, the FeNO angle is 144 degrees with a Fe-NO bond length of 1.87 angstrom. However, when prepared from the reaction of NO with reduced Mb(II), the FeNO angle is 120 degrees with a Fe-NO bond length of 2.13 angstrom. This difference in FeNO conformations as a function of preparative method is reproducible, and suggests a role of the distal pocket in hh Mb(II)NO in stabilizing local FeNO conformational minima. (c) 2006 Elsevier Inc. All rights reserved.